Washing of nitrocellulose and the like



Patented Oct. 30, 1934 UNITED STATES PATENT OFFICE Milton 0. Schur,Berlin, N. H., assignor to Brown Company, Berlin, N. 11., a corporationof Maine No Drawing. Application May 14, 1930, Serial No. 452,522

3 Claims.

This invention in its broadest aspects relates to the recovery of liquidfrom a liquid bath in which porous sheet material is submerged, itsobject being to make possible a practically complete displacement of theliquid from within the pores of the material. Briefly stated, theprocess of the present invention comprehends the removal of the givenliquid from the bath in which the sheets are submerged, While supplyingthereto another liquid capable of forming a distinctlayer in associationwith the bath. This invention more specifically relates to themanufacture of nitrocelluloses, its general object being to makepossible important advantages and economies in this line of manufactureand at the same time to produce a high grade product. In accordancewiththe process of the present invention, sheets of interfelted cellulosefiber are steeped in a bath of suitable nitrating acid, and afternitration has been 'eiiected to the desired degree, the acid bath isprogressively withdrawn at the bottom of the bath while it is beingdisplaced by Water which is delivered gently onto the surface of theacid bath, preferably at the same rate as the rate of withdrawal of theacid. I have found that such a process is advantageous, in that the rateof withdrawal or draining of acid from the bath may be comparativelyrapid, with little mixing of the water and acid at the interface formedbetween the acid and water layers, as the draining is continued untilthe interface between the water and acid layers has disappeared, so thatsubstantially only water is present in the bath along with the sheets.It is further advantageous, in that the cellulose is used in a conditionin which it lends itself to easy handling, storing, and shipment, bothbefore and after nitration. Other advantages of the process will appearas this description proceeds.

The displacement of the nitrating acid by water has been applied inconnection with the nitration of cotton linters, but, in such case,unless the rate of displacement is exceedingly slow, there issubstantial mixing of acid and water across the liquid interface; and,although theacid may be almost entirely displaced from the intersticesof the mass of nitrocellulose, the last portions of acid drained ofi maybe so diluted with water as to make their reclamation for reuse innitration difficult and expensive. 'The proponents of this socalleddisplacement method as applied to cotton linters have recommended a rateof displacement of about two inches in depth of nitrating vessel perhour (see British Patent No. 8,278, to the Thompsons, dated April 9,1903). This means that if the vessel has any substantial depth, say,

24 inches, twelve hours may be consumed in the draining operation. Ininvestigating this method,

I have found that in the preparation of nitrocellulose from cottonlinters there is serious weakening of the last, say, 15% of the aciddrained from the nitrocellulose, even when the rate of displacement isonly two inches per hour. To be sure, practically all of the acid isdisplaced from the nitrocellulose, but the weakening of the last portionof the acid, say the last 15%, makes the reclamation of this portion forreuse in nitration hardly worth While. When the rate of displacement isincreased, the weakening effect becomes more marked; and, when the rateis increased to as high as 30 inches per hour, there is so much mixingand dilution of the acid with water at the liquid interface thatpronounced heating ensues and the yield of nitrocellulose is seriouslydecreased. These troubles with the displacement method as applied tocotton linters probably more than offset the benefits made possiblethereby and account for its not being generally adopted by the industry.

On the other hand, when sheets of cellulose are nitrated, thedisplacement of acid by water may be practised to very much greateradvantage, for not only is the acid displaced completely by Waterfloating on it, but the displacement is effected with comparativelylittle mixing at the interface, even though the rate of draining is ashigh as 30 inches per hour. Such a rapid rate of draining is important,for two reasons, as with slow draining:

(l) The nitrocellulose at the bottom of the nitrating vessel is exposedto the action of the nitrating acid much longer than the upper layers,with the result that the nitrocellulose charge may be of non-uniformcharacter;

(2) The apparatus and the nitrating acid are tied up unduly, so thatmore equipment and acid are necessary for a given production capacity.

The difierence in results of practising the displacement process withsheets of interfelted cellulose fiber as against cellulose in bulk orshreaded condition is probably due to the fact that with the sheets,particularly when placed in the bath vertically and carefully separatedfrom one another, the liquids have comparatively little tendency to mix,probably by reason of the smooth surfaces of the sheets, the straightline flow through the capillary pores in the sheets afiorded by theinterfelted relationship of the fibers and the comparatively narrowclearances between the sheets; whereas, with cellulose in bulk, becauseof the lumpy character of the charge, unless the interface is loweredexceedingly slowly, the draining is more or less irregular and turbulentfrom lump to lump and mixing is apt to ensue. Regardless of what may bethe actual causes underlying the surprising and important results that Irealize by the practice of my invention, the fact remains that thesurfaces of the sheets, the pores therein, and their othercharacteristics comport with a maintenance of a substantiallyundisturbed condition at the interfacebetween the nitrating acid and thewater,so much so thata' substantially unmixed condition of these liquids7 can be maintained at a rate of the movement of the liquid interface ashigh as about 30 inches per hour, as already described. Aside from theadvantages and economies accruing from the actual nitration operation,the feature of using cellulose'in sheet form is of utmost commercialsignificance, particularly when the cellulose employed as a raw materialis wood pulp which has been refined to high alpha cellulose content andpossesses other qualities'rendering it suitable as a base fornitrocellulose manufacture. 'In such case, the pulp manufacturer isequipped to run theprocessed pulp in the form of an aqueous suspensionover .so-called pulp driers, which are built as large units capable ofhandling at low expense as high as 200,01" more, tons of dry fiber perday. These sheets, which'are known as drier sheets, may be of athickness of about .040 inches and otherwise be similar to those beingused currently as a raw materialfor the production of artificial silk bythe viscose process. When used as a raw material for nitration in theprocess of the present invention, the sheets may be supportedvertically' in closely spaced, parallel relation, as in 3 frames; orthey may be suspended as from glass rods or clamps. They should belowered suffi ciently'in the nitrating vessel to permit the bath ofnitrating acid to cover them completely, their spaced relation thuspermitting uniform access of acid to allthe surfaces of the sheets.

, The process of the present invention is applicable tocellulosesuitable for nitration of various origins, including wood pulp andcotton fiber. While, as hereinbefore noted, it is advantageous to usecomparatively thick sheets of unbeaten washing, stabilizing, anddehydrating operations.

This-means easy handling and a compact and convenient form in which tostore and ship the nitrocellulose. It further means that not only maythenitrating equipment be of the simplest kind, but further that theother equipment of the nitrating'pl'ant may be of the same character,and

that labor costs may be relatively low.

The principles of the'present invention are applicable when it isdesired to displace a given a liquid bath in which the sheets aresubmerged with mother liquid iloatable on the given liquid. Forinstance, after the acid bath in which the sheets are suspended has beendisplaced by water to form a water bath, and thesheets have preferablybeen stabilized in the usual manner, i. e., by

boiling in slightly acidulated water, and washed, the water bath may bedisplaced by alcohol to effect a dehydration of the sheets.

I claim:

1. A process of uniformly nitrating cellulose and rapidly removing themixed nitrating acid therefrom, which comprises maintaining integralporous sheets of interfelted cellulose fiber in closely spaced verticalrelation in a bath of the mixed nitrating acid until nitration has beeneffected; progressively draining the acid from the bath while deliveringwater onto the surface of the bath to form a floating water layer, andcontinuing such draining until the interface between the water and acidlayers has disappeared and substantially only water is present in thebath along with the sheets, the surfaces of such sheets, the porestherein, and their other characteristics comporting with a maintenanceof a substantially undisturbed condition at the interface betweensuchliquids and a substantially unmixed condition of the liquids at a rateof move ment of the liquid interface up to about 30 inches per hour,whereby the rapid drainage of said mixed nitrating acid may beeffectedso asto maintain the uniformity of. nitration of said sheets. 7i

2. A process of uniformly nitrating cellulose and rapidly removing themixed nitrating acid therefrom, which comprises maintaining integralporous sheets of interfelted cellulose fiber ina bath of the mixednitrating acid until nitration has been effected, said sheetsbeing inclosely spaced relationship and defining channels between the sheetsdown through which channels the mixed nitrating acid can havesubstantially unobstructeddrainage, progressively draining the acid fromthe bath while delivering water onto the surface of the bath to form afloating water layer, and continuing such draining until the in,-terface between the water and. acid layers has disappeared andsubstantially only water is present in the bath along with the sheets,the surfaces of such sheets, the pores therein, and their othercharacteristics comporting with a maintenance of a substantiallyundisturbedcondition at the interface between such liquids and asubstantially un-. mixed condition of the liquids at a rate of movementof the liquid interface up to about 30 inches per vhour, whereby therapid drainage of said mixed nitrating acid may be effected so as tomaintain the uniformity of nitration ofsaid sheets. I

3. A process of uniformly nitrating cellulose,

rapidly displacing the mixed nitrating acid with ing such draining untilthe interface between the Water and acid layers has disappeared andsubstantially only water is present in the bath along with the sheets,progressively draining water from the bath while delivering alcohol ontothe surface of the bath to form afloatin'g alcohol layer and continuingsuch'draining until the interface between the alcohol and water layershas'disap peared and substantially only alcohol is present IIO in thebath, the surfaces of such sheets, the pores therein, and their othercharacteristics comporting with a maintenance of a substantiallyundisturbed condition at the interface between such liquids and asubstantially unmixed condition of V the liquids at a rate of movementof the liquid interface up to about 30inches per hour, whereby

